I went to install my cooling fan and found the bolts were not long enough.

Apparently the fan I decided to clean and balance wasn't the one that was last on the motor (don't know if it was original or not).

Here are the differences. Does it matter which one I use? Is one a bus fan or do they just vary with year?

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I've seen both styles, and they do use different length bolts. Not sure of the difference as to engines or years.

I think I remember the cone style on the right had a black plastic bullet shaped plug inserted covering the center hole.

It did. I think those are early style as that's what was on my 70 and had the cap. The two 73s I've owned had the flat-nosed version.

Different versions for different engines. 1.7, 2.0 used the same timing marks and the 1.8 used one. As for the flat and the cone I have no answer, other than it was on the shelf.

Flat ones were used on engines with Air Injection pumps.

Jim